An extensive research and development has been conducted on the use of lithium metal, which is capable of realizing high voltage and high energy density, as the negative electrode of non-aqueous electrolyte secondary batteries. This has led to the current commercialization of lithium ion batteries that use a graphite material in the negative electrode, which material reversibly absorbs and desorbs lithium and provides good cycle life and safety.
However, the practical (i.e., useful) capacity of batteries using a graphite material-based negative electrode is approximately 350 mAh/g, which is very close to the 372 mAh/g theoretical capacity of the graphite material. Therefore, as long as a graphite material is used in the negative electrode, it is not feasible to achieve a dramatic increase in capacity. Meanwhile, as more and more portable appliances become available, non-aqueous electrolyte secondary batteries used as the energy source of such appliances are required to have higher capacities. Accordingly, in order to achieve higher capacities, negative electrode materials having a higher capacity than graphite become necessary.
Alloy forming materials (hereinafter referred to as “alloying materials”) containing silicon or tin are currently receiving attention as the materials that offer a higher capacity. Metal elements, such as silicon and tin, are capable of electrochemically absorbing and desorbing lithium ions, thereby enabling a very large capacity charge and discharge in comparison with graphite materials. For example, it is known that silicon has a theoretical discharge capacity of 4199 mAh/g, which is 11 times higher than that of graphite.
When an alloying material absorbs lithium, it forms a lithium alloy, such as a lithium-silicon alloy or a lithium-tin alloy. The formation of a lithium alloy involves a very large expansion caused by the change in its crystal structure. For example, the volume of silicon theoretically expands 4.1-fold when it absorbs lithium to its maximum. As a result, the active material, i.e., the alloyed material, separates and falls off the current collector of the negative electrode, thereby resulting in loss of electrical conduction and a degradation in battery characteristics, particularly high-rate discharge characteristics and charge and discharge (hereinafter referred to as “charge/discharge”) cycle characteristics. In the case of graphite, its volume expands only 1.1-fold, because lithium is intercalated between the layers of graphite (intercalation reaction).
In order to lessen such expansion and obtain higher capacities, the use of a combination of graphite and an alloying material has extensively been attempted. However, when graphite and an alloying material are simply mixed, the alloying material expands in uneven directions in the electrode plate, so that the graphite particles around the alloying material are moved by the stress exerted by the expansion of the alloying material, thereby resulting in separation. Consequently, the electronic conductivity lowers and the high-rate discharge characteristics and charge/discharge cycle characteristics of the resultant battery deteriorate, in the same manner as the negative electrode including an alloy material alone.
Japanese Laid-Open Patent Publication No. 2000-357515 proposes controlling the ratio of the particle size RSi of a silicide to the particle size Rc of a carbon material, i.e., the RSi/Rc ratio, to 1 or less, in order to lessen the impact of large expansion of the alloying material and improve battery characteristics. However, even if such particle size control can lessen the impact of alloy expansion, it cannot suppress the degradation of current collecting property caused by cracking of particles of alloying material and the like. Also, charge/discharge cycles cause particles of alloying material to become cracked, thereby increasing the surface area of the alloy material. Thus, there is also a problem of side reaction, i.e., formation of a coating film on the surface of the alloy. Accordingly, this proposal is not practical.
Japanese Laid-Open Patent Publication No. 2000-243396 proposes embedding, in a carbon particle, a metal particle or a metal oxide particle that is capable of electrochemically reacting with Li. According to this proposal, by fixing the metal particle or metal oxide particle to the surface of the carbon particle, the separation of the metal or metal oxide particle due to its expansion is suppressed. In this case, this proposal is highly effective in the initial stage of charge and discharge cycles, but repetitive expansion and contraction causes the metal particle or metal oxide particle to separate from the carbon particle. As a result, the expansion rate of the negative electrode increases, and separation occurs throughout the electrode plate.
As described above, in order to make full use of a high capacity alloying material as a negative electrode material, the use of a combination of an alloying material and a graphite material has been extensively examined, but no proposal has succeeded in sufficiently reducing the impact of uneven expansion of the alloy material. Specifically, according to conventional proposals, the electrical conduction between particles in a negative electrode is broken, and an alloying material and a graphite material separate from a current collector. Consequently, the electronic conductivity of the negative electrode lowers, leading to a degradation in battery characteristics.